Magnetic recording media are extensively employed in the computer industry and can be locally magnetized by a write transducer or write head to record and store information. The write transducer creates a highly concentrated magnetic field which alternates direction based upon bits of the information being stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the recording medium, grains of the recording medium at that location are magnetized. The grains retain their magnetization after the magnetic field produced by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The magnetization of the recording medium can subsequently produce an electrical response to a read sensor, allowing the stored information to be read.
There is an ever increasing demand for magnetic recording media with higher storage capacity and lower noise. Efforts, therefore, have been made to reduce the space required to magnetically record bits of information while maintaining the integrity of the information. The space necessary to record information in magnetic recording media depends upon the size of transitions between oppositely magnetized areas. It is, therefore, desirable to produce magnetic recording media that will support the smallest transition size possible. However, the signal output from the transition must avoid excessive noise to reliably maintain the integrity of the stored information. Media noise is generally characterized as the sharpness of a signal on readback against the sharpness of a signal on writing and is generally expressed as signal-to-media noise ratio (SMNR).
The increasing demands for higher areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of coercivity (Hc), magnetic saturation (Ms), magnetic remanance (Mr), coercivity squareness (S*), SMNR, and narrow track recording performance. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements.
The linear recording density can be increased by increasing the Hc of the magnetic recording medium, and can be accomplished by decreasing the medium noise, as by maintaining very fine magnetically non-coupled grains. Medium noise in thin films is a dominant factor restricting increased recording density of high density magnetic hard disk drives, and is attributed primarily to inhomogeneous and large grain size and intergranular exchange coupling. Accordingly, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
Longitudinal magnetic recording media containing cobalt (Co) or Co-based alloy magnetic films with a chromium (Cr) or Cr alloy underlayer deposited on a non-magnetic substrate have become the industry standard. For thin film longitudinal magnetic recording media, the desired crystallized structure of the Co and Co alloys is hexagonal close packed (HCP) with uniaxial crystalline anisotropy and a magnetization easy direction along the c-axis is in the plane of the film. The better the in-plane c-axis crystallographic texture, the more suitable is the Co alloy thin film for use in longitudinal recording to achieve high remanance. For very small grain sizes coercivity increases with increased grain size. The large grains, however, result in greater noise. Accordingly, there is a need to achieve high coercivities without the increase in noise associated with large grains. In order to achieve low noise magnetic recording media, the Co alloy thin film should have uniform small grains with grain boundaries capable of magnetically isolating neighboring grains. This type of microstructural and crystallographic control is typically attempted by manipulating the deposition process, grooving the substrate surface and proper use of an underlayer.
Underlayers can strongly influence the crystallographic orientation, the grain size and chemical segregation of the Co alloy grain boundaries. Conventional underlayers include Cr and alloys of Cr with elements such as titanium (Ti), tungsten (W), molybdenum (Mo) and vanadium (V).
There are other basic characteristics of magnetic recording media, aside from SMNR, which are indicative of recording performance, such as half-amplitude pulse width (PW50), overwrite (OW), and modulation level. At high linear recording density, adjacent bits are crowded together. A wide PW50 results in interference which limits the linear packing density of bits in an even track and, hence, reduces packing density in a given area thereby eliminating the recording capacity of the magnetic recording medium. Accordingly, a narrow PW50 is desirable for high areal recording density.
OW is a measure of the ability of a magnetic recording medium to accommodate overwriting of existing data. In other words, OW is a measure of what remains of a first signal after a second signal, e.g., at a different frequency, has been written over it on the medium. OW is considered low or poor when a significant amount of the first signal remains.
It is extremely difficult to obtain optimum performance from a magnetic recording medium by optimizing each of the PW50, OW, SMNR and modulation level, as these performance criteria are interrelated and tend to be offsetting. For example, if coercivity is increased to obtain a narrower PW50, OW is typically adversely impacted, as increasing coercivity typically degrades OW. A thinner medium having a lower Mr x thickness (Mrt) yields a narrower PW50 and better OW; however, the medium signal is typically reduced. Increasing the squareness of the hysteresis loop contributes to a narrower PW50 and better OW; however, noise may increase due to intergranular exchange coupling and magnetostatic interaction. Thus, a formidable challenge is presented to optimize magnetic performance in terms of PW50, OW, SMNR and modulation level.
As the drive to higher and higher recording density increases, attempts have been made to achieve high coercivities by increasing the amount of platinum (Pt) in the Co-based magnetic alloys. In order to improve SMNR, the chromium (Cr) content is simultaneously increased. However, as a result of increasing both the Pt and Cr contents, the cobalt (Co) content is decreased thereby detrimentally impacting Ms, triggering lower amplitudes and weaker signals. In order to compensate for the diminished signal, the magnetic film can be made thicker. However, as the thickness of the magnetic film increases, the PW50 becomes wider and the resolution decreases.
Chen et al. in U.S. Pat. No. 5,763,071 disclose a magnetic recording medium containing a magnetically isotropic layer formed directly on a magnetically anisotropic layer. Zhang in U.S. Pat. Nos. 5,952,097 and 5,772,857 disclose magnetic recording media containing bi-layer magnetic films comprising a lower magnetic layer of a low noise magnetic material and an upper magnetic layer of a high coercivity magnetic material, such as a CoCrTa lower layer and a CoCrTaPt upper layer. Miyazaki et al. in U.S. Pat. No. 5,558,945 disclose a magnetic recording medium containing at least two ferromagnetic thin films, wherein the uppermost film has a higher saturation magnetization and a higher coercive force than the lower film.
There exists a continuing need for high areal density longitudinal magnetic recording media exhibiting high coercivity, high SMNR, high signal, low PW50 and high resolution.